1. Field of the Invention
The invention relates to the production of metal-coated films in a continuous "roll-to-roll" process. The metal-coated films produced in accordance with the invention are distinguished, in relation to other known metal-coated films which can likewise be produced continuously, by two advantages: firstly, they do not include the commonly employed adhesive interlayers between film and metal coat, and, secondly, they can have very thin metal coats of down to 2-8 .mu.m. On the basis of these dual advantages, the films produced in accordance with the invention can be used with particular advantage to produce miniaturized ultrafine conductor circuits and also 2-layer circuits and multilayers.
2. Description of the Related Art
Polyester and polyimide films with copper foils laminated atop them are known and are produced in large quantities for the production of flexible circuits and of film connectors (Jurgen Bednarz, Kunststoffe in der Elektrotechnik und Elektronik Plastics in electrical engineering and electronics!, Verlag W. Kohihammer, 1988, p. 224 ff.). However, these laminates generally include an adhesive interlayer, which is typically from 25 to 50 .mu.m thick and consists, for example, of an epoxy resin or an acrylate adhesive. Moreover, the copper foils used must be of a certain minimum thickness in order to safeguard mechanical handling. In many cases copper foils 18 .mu.m, 35 .mu.m or 70 .mu.m thick are employed. There are also thinner copper foils on the market, for example those with a thickness of 9 .mu.m, whose use might also be considered. Such thin foils additionally possess, however, a treatment which is up to 9 .mu.m thick, and therefore require etching times whose duration is similar to that for the common, thicker copper foils. Owing to the interlayer of adhesive and the fairly high copper layer thicknesses, the production of 2-layer circuits is difficult and costly, and the production of ultrafine conductor circuits is, accordingly, limited in terms of the width of the individual conductor tracks. The adhesive, moreover, becomes smeared in the course of drilling and has to be "etched back" prior to through-hole plating. In addition the adhesive is sensitive to a variety of organic solvents which might act on circuit boards produced in this way. The omission of an adhesive interlayer would have the advantage that the product could be made even thinner and, in particular, it would be possible in the case of multilayer circuit boards to avoid the currently very great expansion of multilayers in the interlayer. It is therefore a concern of the present invention to avoid such disadvantages.
Another composite material which has become known, comprising copper foil/plastic, is produced by taking the known 18 .mu.m or 35 .mu.m thick copper foils already mentioned above and generating thereon a polyimide layer, by pouring on and drying a solution of a soluble polyimide. These products are produced and marketed, for example, under the designation ESPANEX.RTM. by Rogers, USA. Similar products are manufactured by Nippon Steel Chemical Co., Ltd. Relative to the polyimide film/copper foil laminates mentioned further above, products of this kind have the great advantage of containing no interlayer of adhesive. Nevertheless, they are still based only on the 18 or 35 .mu.m thick copper foils, whose mechanical handling properties are still good, and are therefore less well suited to the production of ultrafine conductor circuits.
A further metal-coated film, bearing the designation NOVOCLAD.RTM. from Sheldal, USA, consists of a polyimide film to which copper has been applied by sputtering. This product possesses the advantage of a very thin copper layer, in the context of the production of ultrafine circuit boards, however, the adhesion of the copper layer is rather low.
EP 256 395 (equivalent to U.S. Pat. No. 5,182,135) and EP 322 641 (equivalent to U.S. Pat. No. 4,910,045) describe processes for the firmly adhering deposition of metal coats atop polymer surfaces, for example plastics films. For this purpose, metallizable primers in the form of binder-containing activator formulations are applied to plastics substrates, and, after drying at elevated temperatures, are provided in chemical metallizing baths with a firmly adhering metal coat. These processes are extremely well suited to the discontinuous production of flexible circuits and ultrafine conductor circuits; however, they are unsuitable for a continuous process, for the following reasons. For the production of special flexible circuits it is true that the deposition of a copper layer having a thickness of 2 .mu.m is sufficient. In order, however, to deposit such a copper layer of 2 .mu.m, a residence time in a chemical copper bath of about I hour is necessary. Such long residence times makes it impossible to transfer the technique to a continuous process, since it would be necessary to have either a much too low rate of passage through the metallizing bath or else far too great spatial dimensions of this bath.